1. Field of the Invention
This invention relates to photodetectorsxe2x80x94in particular to an integrated multiple-wavelength detector array, which is programmable.
2. Description of the Related Art
The applications for high performance infrared (IR) photodetectors are wide ranging. These include missile seekers, infrared astronomy, infrared cameras for surveillance and industrial process monitoring, ellipsometry and environmental monitoring. Many such photonic and sensor applications require a wide wavelength range. However, photodetectors have a limited wavelength of operation. FIG. 1 shows the wavelength spectra of different types of existing photodetectors and spectral requirements of different types of applications. The wavelength limitation of photodetectors is due to the fact that photodetection in semiconductors is strongly dependent on the energy band of the semiconductor material. Therefore, in order to achieve multi-wavelength operation of sensors, it is necessary to have different types of semiconductor material on the same substrate. Furthermore, in order to maximize the utility of such detectors, it is desirable to be able to electronically control the window of operation of the detector array and achieve a type of electronically controlled optical fiber. It is clear from FIG. 1 that successful development of a multi-wavelength detector will require the choice of an optimum combination of suitable materials and a technique of growing suitable epitaxial wafers having multiple material structures for the desired wavelength spectra.
Although high performance single wavelength detectors have been developed and produced, a viable approach to combining them on the same substrate for multi-wavelength operation has not been developed. The materials typically used for photodetectors such as CdS, Si and GaAs at the shorter wavelengths and HgCdTe (MCT), GaInAs, Ge, InAs and InSb for long wavelengths cannot be easily combined on the same substrates and meet the requirements outlined above. Thus while short wavelength materials like GaAs and Si can be produced with large substrate areas, they cannot easily be integrated with MCT, InSb and other long wavelength material. In addition, the most commonly used infrared detector materials like MCT, InAs and InSb are not necessarily the most optimum to use on account of poor uniformity, poor mechanical and chemical properties and sensitivity to heat during device fabrication, a well as the unavailability of large substrates. Furthermore, the resistivity of such substrates is low, making it difficult to integrate electronic functions with the detector.
Recent advances in detector technology has resulted in the development of quantum infrared photodetectors (QWIP) based on intersubband transitions in AlGaAs/GaAs, AlInAss/GaInAs, InP/GaInAs and GaInAsP/GaInAs superlattices. These new classes of detectors have been demonstrated with performance comparable to traditional infrared detector materials like MCT. Unlike the latter, they can be fabricated on large area substrate with high yields and improved uniformity. The dominant QWIP approach is based on the AlGaAs/GaAs heterostructure, the operation of which is limited to mid- to far infrared wavelengths (i.e.  greater than 5 xcexc)because the bandgap discontinuity of the optimum AlGaAs/GaAs heterojunction is low. The AlInAs/GaInAs heterojunction has large bandgap discontinuity suitable for shorter wavelength QWIP operation, however it is not optimum for longer wavelength operation. It is not desirable to combine these two types of heterojunction on the same substrate because of the large lattice mismatch between the two material systems.
Non-programmable two color detectors operating in mid-wave (MW)xe2x80x94long wave (LW) or long-wave (LW)xe2x80x94long wave (LW) ranges have been proposed and demonstrated. However, the integration of programmable three color detectors so far has not been proposed.
An object of this invention is to integrate photodetectors for different wavelengths in a monolithic structure. Another object of this invention is to integrate photodetectors of different wavelengths as an active matrix array. Still another object of this invention is to integrate programmable electronics to address each detector in a tri-color photodetector array. A further object of this invention is to provide an integrated tri-color photodetector which is durable and reliable.
These objects are achieved by the choice of an optimum combination of suitable materials and a technique for growing suitable epitaxial wafers having material structure for the desired wavelength spectra. Photodetectors for different wavelengths are integrated on a common substrate. These detectors are multiplexed for sensing the different wavelengths. Peripheral electronics for multiplexing can also be integrated on the same substrate.
Three photodetectors of different wavelengths can form a pixel of an active matrix array with hundreds of rows and columns.